Hydrogeophysical characterization and monitoring of a peatland in the Hautes Fagnes

Details

Supervisors

Lambot, Sébastien ; Henrion, Maud

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en sciences et technologies de l'environnement, à finalité spécialisée

Abstract

Although they represent only 3% of the land surface, peatlands contain almost 30% of the world's soil carbon stock. Because of their high organic content, peatlands are important carbon stores. Peatlands have high-water storage capacity, which allows them to function as water reservoirs. The hydraulic properties of peatlands therefore play a key role in maintaining conditions favourable to carbon accumulation. Peatlands have a regulatory function and a water regulation function. It has also an essential role in biodiversity. In critical zone research, the integrated study of peatland remains a key knowledge gap, especially in Belgium. This master's thesis aims at addressing this problem by studying hydraulic parameters in a hillslope peatland in the Hautes Fagnes. To collect the data needed, several methods were used: an in-situ method, namely the TDR (Time Domain Reflectometry) probes and a laboratory method, the pressure cell apparatus. Direct modelling allows to integrate these parameters into a Hydrus model. Soil water retention curves have been established and values of the model fitting parameters are obtained with very high coefficients of determination for both soils. Unsaturated hydraulic parameters in peat soils have been successfully estimated using the van Genuchten-Mualem unimodal model via the RETC program. Clay bulk density, with an average value of 1.2 [g/cm3 ], corresponds to loamy clay and an increase of the bulk density with depth is visible. The average value of bulk density of peat is equal to 0.2 [g/cm3] with no relationship between depth and density. The clay samples have an average value of the volumetric water content (VWC) at saturation (θs) of 0.59 which looks like very fine-textured clay soil. For peat samples, this value is 0.94. The values of these hydraulic parameters for peat confirm the great water storage capacity of the studied peatland. Several TDR probes were placed along the slope at 5 locations and 3 depths each. This enabled a long-term study of the VWC. The variations observed are closely linked to precipitation. Along the study transect, the position of the probes has no influence on the evolution of the VWC. The presence of the water table is assumed to be between 30 and 50 cm depth. Therefore, peatland is an almost saturated environment that offers little absorption capacity. Any excess precipitation can not infiltrate, but only run off downhill through the surface vegetation. There is a threshold for rapid flow at very shallow depths in peatlands. Below this threshold, the peat is nearly impermeable and lateral/horizontal water movement is slow. In direct modelling part, the simulations roughly match the data, but tend to be more sensitive to boundary conditions and to have larger amplitudes at 30 cm depth. At a depth of 10 cm, the best results are observed, although in some cases the simulation and TDR data do not match.